This invention relates to devices for generating intense ion beams and more particularly to a device for efficiently generating an intense unidirectional ion beam.
Heretofore different types of devices have been used to produce intense, pulsed ion beams. These devices fall primarily in three different categories: pinched beam diodes, magnetically insulated diodes, and reflex triodes. Pinched beam diodes are efficient but cannot be operated in an external magnetic field. In magnetically insulated diodes, the ions are accelerated perpendicularly to an external magnetic field which must exceed the value required to suppress the electron flow. These ion sources are characterized by high efficiences but low ion-current density.
Reflex triodes can be used in an external axial magnetic field, and they have been used to produce beams with high ion-current densities. They can produce solid or annular beams of various cross sections. However, the ion flow in reflex triode is bidirectional i.e., approximately the same number of ions flow back toward the real cathode (and hence cannot be used) as flow forward toward the virtual cathode. Partly because of the backward-flowing ions, which enhance emission of electrons from the real cathode, the triode suffers from an abrupt decrease in impedance. The bidirectionality of thee beam and the impedance-collapse problem limit the efficiency of the reflex triode to a relatively low value.
The reflex tetrode has all the advantages of the reflex triode. Furthermore, the unidirectionality of the ion beam and the significant decrease in impedance collapse allow the reflex tetrode's efficiency to be much larger than that of the reflex triode.